1. Field of the Invention
This invention relates to improved dentifrice abrasives. More particularly, the present invention relates to novel, high fluoride compatible amorphous precipitated silica abrasives which are suitable for use in therapeutic toothpaste compositions containing soluble fluoride salts. The invention further relates to methods for preparing these improved precipitated silica abrasives and to toothpastes containing the improved abrasives including toothpaste embodiments. Such toothpaste compositions exhibit both high fluoride compatibility and high cleaning performance.
2. The Prior Art
The function of an abrasive substance in formulations intended for use in the oral cavity is to remove various deposits, including pellicle film, from the surface of the teeth. Pellicle film is tightly adherent and often contains brown or yellow pigments and thus imparts an unsightly appearance to the teeth. An advantageous toothpaste abrasive material should maximize film removal without causing undue abrasion to the hard tooth tissue. Dental researchers are continually concerned with developing toothpaste abrasives which demonstrate satisfactory levels of cleaning and which are not unduly abrasive and damaging to the oral tissue.
In addition to abrasives, therapeutic toothpastes typically contain fluoride ion sources. The beneficial reduction in the incidence of dental caries resulting from the topical application to dental enamel surfaces of solutions containing fluoride ions is well known. Especially at solution pH's between about 4 and 8, fluoride ions are believed to interact with enamel to reduce the acid solubility of such enamel. Enamel so treated with fluoride is more resistant to the formation of dental caries. Accordingly, therapeutic toothpaste compositions are formulated to provide fluoride ion availability in brushing solutions formed in the oral cavity during use.
It has been postulated that the effectiveness of fluoride treatment in providing enamel antisolubility/anticariogenic benefits is dependent upon the amount of fluoride ion which is available for uptake by the enamel being treated. It is, of course, therefore desirable to formulate toothpaste compositions which provide maximum fluoride ion availability in brushing solutions formed therefrom. However, efforts to utilize such ionic fluoride anticariogenic agents in toothpastes suitable for home use have been unable to provide the theoretical maximum soluble fluoride because of the tendency for ionic fluoride to be inactivated and thereby rendered unavailable for enamel uptake. That is, the toothpastes lose, upon storage (at rates which increase with temperature), the capability of providing the theoretical maximum amount of soluble fluoride. For purposes of this invention, the "soluble fluoride" content of any given toothpaste composition refers to the ppm concentration of fluoride ion which is found in the supernatant sample centrifuged from 1:3 by weight slurry of the toothpaste in water (1:3=toothpaste:water).
Fluoride ion sources tend to interact with toothpaste impurities and with such toothpaste components as abrasives, buffers, etc. Such interaction diminishes the ability of the fluoride source to provide "soluble fluoride" upon use. The propensity of the toothpaste compositions herein to maintain their levels of soluble fluoride after storage is expressed hereinafter as "toothpaste fluoride compatibility". Thus, the toothpaste fluoride compatibility of a particular toothpaste composition is that percentage of the theoretical maximum amount of fluoride source that is actually measured as soluble fluoride after storage for a specified time and at a specified temperature (e.g. one week at 120.degree. F.). Similarly, the propensity of such a dentifrice component such as the abrasive to interact with the fluoride source to diminish the measured "soluble fluoride" level from the theoretical maximum amount of fluoride source (particularly in the presence of pellicle film penetration agents) is expressed as "abrasive fluoride compatibility". The test procedures used herein to determine "toothpaste fluoride compatibility" values and "abrasive fluoride compatibility" values are described more fully hereinafter.
One toothpaste component which can pose special difficulties in formulating fluoride toothpastes is a precipitated silica abrasive component. Precipitated silica abrasives are desirable for use in toothpastes since they have desirably low dentin abrasion values. Certain prior art precipitated silica abrasives are generally compatible with soluble fluoride sources but have insufficiently high abrasivity to provide effective cleaning performance. Certain other prior art precipitated silica abrasives provide acceptable cleaning performance but have low abrasive fluoride compatibility as measured by the method hereinafter. There is a clean need to formulate precipitated silica abrasives which exhibit high "abrasive fluoride compatibility" as well as acceptable cleaning performance. Accordingly, it is an object of the present invention to provide precipitated silica abrasives which exhibit high "abrasive fluoride compatibility" as well as acceptable cleaning performance.
It is of course well known that therapeutic toothpaste compositions contain calcium phosphate materials as abrasives. These calcium materials are present in large amounts as described above and illustrated for example in U.S. Pat. No. 3,624,199, issued Nov. 30, 1971, Norfleet et al, and U.S. Pat. No. 3,864,471, issued Feb. 4, 1975, Mills et al. Toothpaste compositions are also known in the art which contain small amounts of alkaline earth metal ions, such as calcium ions, and compositions of this type are illustrated by U.S. Pat. No. 3,991,177, issued Nov. 9, 1976, Vidra et al. This patent discloses toothpaste compositions which contain a stabilizer-activator for a dextranase enzyme agent with the stabilizer-activator being a salt such as calcium chloride present in an amount of 0.001 to 0.3 weight percent. This composition can also contain therapeutic fluoride and the abrasive agent is calcium carbonate.
Other prior art which discloses toothpaste compositions containing alkaline earth metal compounds or ions include U.S. Pat. No. 3,095,356, issued Jun. 25, 1963, to Moss; U.S. Pat. No. 3,122,483, issued Feb. 25, 1964, to Rosenthal; U.S. Pat. No. 3,669,221, issued Jun. 13, 1972, to Hase; U.S. Pat. No. 3,782,446, issued Jan. 1, 1974, to Walter; U.S. Pat. No. 3,842,168, issued Oct. 15, 1974, to Colodney; and, U.S. Pat. No. 3,689,537, issued Sep. 5, 1972, to Kuder.